1. Field of the Invention
The present invention relates to solid state image sensors having a plurality of light-shielding layers, and manufacturing methods of such sensors.
2. Description of the Related Prior Art
One recent means for making a solid state image sensor colorable is a filter bonding method which bonds a color film layer formed on a plate glass to a solid state image sensing device. However, to deal with more complicated manufacturing processes and further advanced devices' minuteness, the above method has gradually been superseded by a so-called on-chip color filter method which forms a color filter layer directly on a semiconductor substrate carrying the solid state image sensing device. In addition, especially for a compact solid state image sensor, a micro lens, which focuses incident light onto a light-receiving portion, is mounted on the color filter layer, improving light-receiving sensitivity.
A description will now be given of conventional solid state image sensors. FIG. 23 shows a sectional view of a principal part of the conventional solid state image sensor, giving an example of the color filter bonding method.
A semiconductor substrate 1 carrying the solid state image sensing device has a terribly rough surface; accordingly, the surface is first smoothed by an underlying smooth layer 4 and then a transparent layer 6 is mounted on it, Subsequently, a color filter 13 is bonded onto it via a color filter bonding layer 12. A light-shielding layer 14, which comprises a light-absorption layer or a light-reflection layer formed on a plate glass surface of the color filter 13, is arranged so as to coat a vertical CCD 10 and shield stray light (referred to hereinafter as flare light) from entering a photodiode 2. Incident light 8 transmitting through an optical system of a video camera reaches the photodiode 2 via the color filter 13, and its optical energy is converted into an electric signal.
FIGS. 24 and 25 each show sectional views of principal parts of other conventional solid state image sensors, taking examples of the on-chip filter method.
In general, according to the on-chip filter method, the underlying smooth layer 4, a light-shielding layer 5, a transparent film 6, and a focus lens 11 (referred to hereinafter as an on-chip lens) are stacked, as shown in FIG. 24, on the semiconductor substrate 1 which carries the solid state image sensing device. The incident light 8 focused by the on-chip lens 11 is thus efficiently led to the photodiode 2. The light-shielding layer 5 formed in the transparent film 6 shields flare light as internal reflection light 9 from being taken as spurious signals in the photodiode 2.
FIG. 25 shows a solid state image sensor which has the light-shielding layer 5 thicker than that of the conventional solid state image sensor shown in FIG. 24, and is designed to more completely shield flare light by controlling the thickness. However, the above conventional constructions have the following disadvantages, and thus cannot shield flare light satisfactorily:
According to the solid state image sensor employing the filter bonding method shown in FIG. 23, an obliquely entering beam of the incident light 8 is reflected by the light-shielding film 3, then repeatedly reflected on the color filter 13 and/or in the transparent film 6 and finally, unexpectedly reaches the photodiode 2 only to cause a spurious signal.
According to the solid state image sensor employing the on-chip filter method shown in FIG. 24, the light-shielding layer 5 cannot satisfactorily absorb and shield, due to its thin single layer construction, flare light generated from the obliquely entering light 8; the flare light is repeatedly reflected in the shielding film 3 or the like, and unexpectedly reaches the photodiode 2 only to cause a spurious signal.
On the other hand, the solid state image sensor employing the on-chip filter method shown in FIG. 25 can successfully absorb and shield the obliquely entering light 8 due to the thick light-shielding layer 5 of a light-absorption layer. Nevertheless, a beam of the incident light 8 which has transmitted through the circumference of the on-chip lens 11 is shielded by the thick light-shielding layer 5; as a consequence, even light expected to enter the photodiode 2 is absorbed, which lowers sensitivity. Besides, in an attempt to form the light-shielding layer 5 by photolithography, a negative type of natural protein material, generally used for the lithography, cannot be fully resolved as the light-shielding layer 5 requires a thicker film thickness, which facilitates the film remainder of the light-shielding layer 5. Consequently, the photodiode 2 is partially covered at its circumference, and lowered in sensitivity. Further, the thick light-shielding layer 5 sometimes induces material to be melted in a development process subsequent to an exposure process, and causes a blotted image.
The present invention overcomes the above conventional disadvantages; it is an object of the present invention to provide a solid state image sensor and a manufacturing method of it, which sensor forms high quality of images by successfully shielding flare light generated from obliquely entering light and internal reflection light, and eliminating spurious signals.